1. Field of the Invention
The present invention relates to a synchronous rectifying DC-DC converter and, in particular, to dead time control suitable for high-frequency switching.
2. Description of the Related Art
As portable devices become multifunctional, more and more power supply circuits with different operating voltages are incorporated into the portable devices. For example, a cellular phone includes power supplies such as those for a baseband IC, LCD driver, and power amplifier module with different operating voltages. As power supply circuits for converting a voltage supplied from a battery to an operating voltage of each power supply circuit, synchronous rectifying DC-DC converters have been known in which a switching transistor and a commutating transistor are connected in series between an input power supply and a ground and are turned on and off in a complementary manner to supply a DC voltage to a capacitor of a smoothing circuit connected in parallel with the commutating transistor. In the synchronous rectifying DC-DC converters, power efficiency is increased by controlling dead time so as to prevent a short-circuit current from flowing through the switching transistor and commutating transistor, as disclosed in Japanese Patent Laid-Open No. 2001-112241, for example. Examples of dead time control methods conventionally used include a method in which a certain delay time is inserted in a gate signal controlling the drive of the switching transistor and commutating transistor and alternatively a method in which an output voltage of an error amplifier is shifted to produce a dead time according to the amount of the shift.
The dead time control method described above is feasible when there is a sufficient time in a low-frequency range, and may improve power efficiency to some extent by optimization of dead time. However, a trend in DC-DC converter design for portable devices is that inductance and capacitor constants are reduced by increasing switching frequency, thereby reducing the size of the components. It is predicted that switching operation at 10 MHz or higher will be achieved in the near future. In such a high-frequency range, there will be little time available for dead time control.
In addition to the circumstances, there is another trend that variations in typical values of parameters (such as the DC resistance of an inductor, the on-resistance of a PMOS transistor, and the switching period of an oscillator) of the DC-DC converter are increasing. Therefore, there is a demand for development of a dead time control method capable of strictly controlling the drive of a commutating transistor with a high degree of accuracy by taking into consideration variations in parameters specific to individual products.
For example, if the lower limit of the input voltage of an internal battery (lithium ion battery) of a portable device is 2.8 V, the output voltage of the DC-DC converter is 1.8 V, the upper limit of output current is 1.2 A, the DC resistance of the inductor is 120 mΩ, the on-resistance of the MOS transistor is 350 mΩ, and the maximum error rate of switching frequency is 15%, then the allowable dead time is limited to 15 nsec or less. It is very difficult for the conventional dead time control methods, which rely on a static approach, to perform dead time control within such a severely limited time.
Therefore, an object of the present invention is to provide a dead time control method that identifies the allowable margin of dead time that is specific to a synchronous rectifying DC-DC converter, recognizes a critical situation in which a commutating transistor cannot be turned on for a reason such as a temporary variation in an output voltage of the synchronous rectifying DC-DC converter, and adaptively prevents the commutating transistor from being turned on in the critical situation.